Creating Logs

One way to debug apps is by logging information in the code. This enables seeing which code was run when a problem occurred and what the value of variables were. You can directly call GLib.Log () or use the convenience functions listed below.

To view logs from all your applications you can use journalctl.

Debug

Debug logs usually give detailed information on the flow through the system and are not printed to Terminal or logs by default.

Log functions, like debug use printf style formatting and can be called like this:

string name = "Bob";
int age = 30;
debug ("Person: %s %i", name, age);

Start your app with G_MESSAGES_DEBUG=all to print debug messages

Info

Use info log level to log informational messages as well as interesting runtime events. These logs are also immediately visible on a status console, and should be kept to a minimum.

info ("An event occurred");

Message

Use the message log level to output a message.

message ("An event occurred");

Warning

The warning log level outputs messages that warn of, for example, use of deprecated APIs, 'almost' errors, or runtime situations that are undesirable or unexpected, but not necessarily "wrong". These logs are immediately visible on a status console.

warning ("Something potentially problematic happened!");

Start your app with G_DEBUG=fatal-warnings to exit the program at the first call to warning () or critical ()

Critical

Critical log level is used when there is a severe application failure that should be investigated immediately.

critical ("A major issue occurred! Uh oh!");

Start your app with G_DEBUG=fatal-criticals to exit the program at the first call to critical ()

Error

Error log level includes logs for runtime errors or unexpected conditions. These errors are immediately visible on a status console and cause your app to exit.

error ("Some catastrophic happened and the app had to exit!");